This invention relates in general to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of counter-rotated horizontal casting rolls that are cooled so that metal shells solidify on the moving roll surfaces, and are brought together at a nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle or series of delivery nozzles (also called the “core nozzles”) located above the nip, forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the casting rolls so as to dam the two ends of the casting pool against outflow.
Further, the twin roll caster may be capable of continuously producing cast strip from molten steel through a sequence of ladles. Pouring the molten metal from the ladle into a smaller vessel before flowing through the metal delivery nozzle enables the exchange of an empty ladle with a full ladle without disrupting the production of cast strip.
During operation, in order to inhibit certain defects from occurring in the cast strip, it is important to maintain certain desired conditions of the molten metal in the casting pool, including temperature, composition and flow rate. Particularly important to casting quality thin strip is controlling the flow rate and molten metal temperature in the area where the side dams, casting rolls and meniscus of the casting pool intersect, the “triple point” area or region. The formation of pieces of solid metal known as “skulls” in the casting pool in the vicinity of the confining side plates or dams have been observed. The rate of heat loss from the casting pool is higher near the side dams (called the “triple point region”) due to conductive heat transfer through the side dams to the casting roll ends. This localized heat loss near the side dams has a tendency to form “skulls” of solid metal in that region, which can grow to a considerable size and fall between the casting rolls and causing defects in the cast strip. An increased flow of molten metal to these “triple point” regions, the regions near the side dams, have been provided by separate direct flows of molten metal to these triple point regions. Examples of such proposals may be seen in U.S. Pat. No. 4,694,887 and in U.S. Pat. No. 5,221,511. Increased heat input to these triple point regions has inhibited formation of skulls.
To control flow in the triple area, the distance between the side dams and the ends of the delivery nozzles nearest the side dams should be controlled and maintained substantially constant. This distance has been found so sensitive that even compensation for wear of the side dams and the delivery nozzles needs to addressed. These components typically wear at different rates. The approach in the past has been to provide a common support for each side dam and adjacent portion of the delivery nozzle. Coupling of the positioning and support for the delivery nozzles and side dams enabled the distance between the side dams and nearest end of a delivery nozzle to be maintained.
Apparatus and method for controlling and maintaining a set distance between the outer ends of the delivery nozzles and the side dams during a campaign is disclosed in U.S. Pat. Nos. 6,910,523, 6,588,492, 7,147,035. The apparatus and method disclosed has a carriage assembly for commonly supporting the side dams and nearest delivery nozzles to maintain distance between the side dams and ends of the delivery nozzles at a set distance with wear of the side dams. This common support was believed important to maintain the distance between the side dam and end of the delivery nozzle. Although the delivery nozzles could be moved relative to the side dams by the carriage assembly, the movement also involved simultaneously moving of both delivery nozzle and the adjacent side dam to maintain the distance between the side dam and end of the delivery nozzle. This movement affects the side dam force and thus side dam wear. Moreover, the movement of the side dam by the support to compensate for wear of the side dam required repositioning of the delivery nozzle to maintain the distance between the side dam and the end of the nearest delivery nozzle.
We have found that quality of thin strip casting particularly with control of “skulls” in the “triple point” can be improved by entirely different approach with separate segregated control of each of the side dams and each adjacent delivery nozzle during a casting campaign. The distance between the side dams and the nozzle may be continually varied if desired. Accordingly, we have disclosed a method for casting metal strip comprising:
(a) assembling a pair of counter-rotatable casting rolls to form a nip there between through which thin strip can be cast, and a pair of confining side dams adjacent the ends of the casting capable of supporting a casting pool of molten metal formed on the casting surfaces above the nip,
(b) assembling an elongated metal delivery nozzle with a plurality of moveable metal delivery nozzles disposed axially along and above the nip and capable of discharging molten metal to form the casting pool supported on the casting supports of the casting rolls,
(c) assembling delivery nozzle actuators each capable of axial movement of the delivery nozzles relative to the adjacent side dam separate from the movement of the adjacent side dam, and side dam actuators each capable of axial movement of the side dams separate from the movement of the delivery nozzles during casting, and
(d) controlling a desired distance between the delivery nozzles and the side dams by the axial movement of the delivery nozzle actuators and side dam actuators.
The method of continuously casting metal strip may have only a pair of delivery nozzles. In this embodiment, there is two delivery nozzles arranged end-to-end, and two side dams arranged adjacent the outside ends of the two delivery nozzles along the nip between the casting rolls. The delivery nozzle actuators and side dam actuators are positioned adjacent the ends of the casting rolls to provide axial movement of the delivery nozzles and the side dams along the direction of the nip between the casting rolls.
The method of continuously casting metal strip may further comprise the following steps:
(e) positioning sensors to sense the positions of the side dams and of the delivery nozzles nearest the side dams, and produce electrical signals indicative of said positions of the side dams and of the delivery nozzles nearest the side dams positions,
(f) controlling the positions of the side dams and of the delivery nozzles nearest the side dams responsive to said electrical signals produced by the sensors so as to adjust the positions of the side dams and of the delivery nozzles nearest the side dams responsive to wear of said the side dams and of the delivery nozzles nearest the side dams.
Alternatively or in addition, the method of continuously casting metal strip may comprise forming a groove in each side dam and controlling the depth of the groove in each side dam during a casting campaign by wear from molten metal. This may be done by controlling force exerted by the side dame actuators.
As disclosed is an apparatus for continuously casting metal strip comprising:
(a) a pair of counter-rotatable casting rolls laterally positioned to form a nip there between through which thin strip can be cast, and a pair of confining side dams adjacent the ends of the casting capable of supporting a casting pool of molten metal formed on the casting surfaces above the nip,
(b) an elongated metal delivery nozzle with a plurality of moveable metal delivery nozzles disposed axially along and above the nip and capable of discharging molten metal to form the casting pool supported on the casting supports of the casting rolls,
(c) delivery nozzle actuators each capable of axial movement of the delivery nozzles relative to the adjacent side dam separate from the movement of the adjacent side dam,
(d) side dam actuators each capable of axial movement of the side dams separate from the movement of the delivery nozzles during casting, and
(e) a control system capable of actuating delivery nozzle actuators and actuating delivery nozzle actuators to control the distances between the side dams and the delivery nozzles nearest the side dams by separate axial movement of the delivery nozzle actuators and side dam actuators.
The apparatus for continuously casting metal strip may be only a pair of delivery nozzles. In this embodiment, there is two delivery nozzles arranged end-to-end, and two side dams arranged adjacent the outside ends of the two delivery nozzles along the nip between the casting rolls. The delivery nozzle actuators and side dam actuators are positioned adjacent the ends of the casting rolls to provide axial movement of the delivery nozzles and the side dams along the direction of the nip between the casting rolls.
The apparatus for continuously casting metal strip may further comprise:
(f) sensors capable of sensing the positions of the side dams and the positions of the delivery nozzles nearest the side dams, and produce electrical signals indicative of said positions of the side dams and of the delivery nozzles nearest the side dams positions, to the control system,
(g) where the control system is capable of controlling the positions of the side dams and the positions of the delivery nozzles nearest the side dams responsive to said electrical signals produced by the sensors so as to adjust the positions of the side dams and of the delivery nozzles nearest the side dams responsive to wear of said the side dams and of the delivery nozzles nearest the side dams.
The apparatus for continuously casting metal strip further a control system controls the side dam actuators to cause a groove to be formed in each side dam to controlled the depth during a casting campaign by wear from molten metal.
In either of method for continuously casting metal strip or the apparatus for continuous casting metal strip, the delivery nozzle actuators and side dam actuators may be selected from the group consisting of servo-mechanisms, hydraulic mechanisms, pneumatic mechanisms, gear mechanisms, cog actuators, drive chain mechanisms, pulley and cable mechanisms, drive screw mechanisms, jack actuators, rack and pinion mechanisms, electro-mechanical actuators, electric motors, linear actuators, and rotating actuators.
Various aspects of this invention will become apparent from the following detailed description and accompanying drawings.